The present invention relates to sensors for measuring the temperature and/or the concentration of a gas or of a gaseous free radical.
In particular, the invention finds an advantageous application in the aviation and motor industries as an on-board sensor for measuring temperature and/or concentration, in particular of combustion gases in the field of stationary gas turbines for measuring temperature and/or concentration in combustion chambers.
Conventional pyrometry makes use of two principles for measuring temperature. The first principle relies on the convective effect which heats a sensitive element of the thermocouple or resistance probe type. The element is protected inside an envelope that is generally made of metal and that is very sensitive to corrosion at high temperatures. Its lifetime in a flame is very limited. The second principle consists in measuring the black body type radiation emitted by a surface. Radiation varies in application of Planck""s function and depends on the emissivity of the material which lies in the range 0 to 1. That method measures only the surface temperature of an object. Those methods do not make it possible to measure directly and reliably the temperature of hot gases in a flame.
An object of the invention is to propose a system which does not present those drawbacks.
It is known that the spectral response of a gas or of a gaseous free radical is a function of its temperature and of its concentration.
However, for reasons of cost and of complexity of implementation, it is not possible to envisage using the spectrometers conventionally found on the market as sensors of the temperature or the concentration of a gas.
It is already known, in particular from DE 43 06 645 (U.S. Pat. No. 5,551,780), to measure the temperature of a combustion flame by means of two photodetectors observing the radiation of the flame in two aiming directions and over a wide band of wavelengths.
Nevertheless, the technique described in DE 43 05 645 is difficult to implement since it assumes that two photodetectors can be placed in two corners of the combustion chamber.
Furthermore, that technique is designed to enable temperature measurement to be performed on the combustion gases of a coal boiler, but does not enable temperature measurements to be collected or flame profiles to be determined with accuracy as high as that which is desirable, for example in the field of aviation.
Also known, from JP 60 129 524, is a device for monitoring the temperature of a flame in which optical sensors are used to measure the intensity of the spectrum which is radiated by a surface which is facing said sensors and which is heated by the flame.
That device does not in any way determine the emission spectrum of the combustion gas and it does not make high accuracy measurement possible.
The invention proposes a sensor for measuring the temperature and/or the concentration of a gas or of a gaseous free radical, the sensor comprising photodetector-forming means and calculation means which determine at least one temperature and/or concentration as a function of measurements performed by said photodetectors, wherein said photodetector means are juxtaposed inside a case in which there are also disposed fixed dispersion means for receiving radiation and for dispersing it as a function of wavelength over said photodetectors which take simultaneous measurements of said radiation, the case also having an optical head enabling radiation emitted by said gas or by said radical to be conveyed to said dispersion means, the combination of dispersion means and of photodetectors making it possible to obtain wavelength resolution of less than 0.1%, the number of photodetectors being not less than three.
Such a sensor constitutes a system which is completely passive, unlike Raman type measurement techniques which excite the gases by means of a laser. The system is therefore not intrusive.
It is very simple and of low cost, while being highly reliable, since it detects and analyses radiation from a gas in a given spectral band by using static means without any moving parts.
It constitutes an assembly which is compact and easy to integrate, in particular on a turbine or a combustion chamber.
The combination of the dispersion means and of the photodetectors enables wavelength resolution of smaller than 0.1% to be obtained, thereby making it possible for each detector to have access to spectral information that is very fine, e.g. around a rotation-vibration transition wavelength of the combustion gas molecule, where the spectrum of the gas is, over a narrow wavelength range, highly sensitive to temperature variations.
For example, when using CO2, the measurements performed by the photodetectors lie in the range 2380 cmxe2x88x921 to 2400 cmxe2x88x921.
In addition, when three or more sensors are available, it is possible to have spectral xe2x80x9ccurvesxe2x80x9d which can, for example, be compared with theoretical curves stored in the calculation means. It will be understood that by making a comparison between curves it is possible to obtain much more information than when using only one or two photodetectors measuring the intensity of a spectrum.
Thus, by having a very good model of the spectral region over which the sensor is responsive, and by calculating the radiation budget, the sensor can operate practically in real time (calculation time less than 5 seconds) to produce temperature profiles over about 50 layers of gas, each time comparing the measured spectrum with the calculated spectrum.
Such a sensor can be used to take measurements over very wide ranges of pressure and temperature. It is thus possible to take measurements on hot (350xc2x0 C.) exhaust gases at atmospheric pressure, all the way to combustion flames which correspond to temperatures of more than 1600xc2x0 C. and to pressures of 20 bars.
The means for dispersing the radiation on the photodetectors preferably comprise a diffraction grating.
Advantageously, the diffraction grating is shaped to be generally concave, thereby avoiding the need to add concave mirrors for collimation purposes as would be required with a plane diffraction grating.
By using optical fibers that are transparent in the spectral band under analysis, it is possible to offset measurement proper by as much as several tens of meters.